Isoflavones are crystalline ketones found primarily in leguminous plants. One of the most important sources of isoflavones is the soybean, which contains twelve distinct isoflavones: genistein, genistin, 6″-O-malonylgenistin, 6″-O-acetylgenistin, daidzein, daidzin, 6″-O-malonyldaidzin, 6″-O-acetyldaidzin, glycitein, glycitin, 6″-O-malonylglycitin, 6″-O-acetylglycitin (Kudou, Agric. Biol. Chem. 55, 2227-2233, 1991). These soybean isoflavones share the generic structure shown below:

Dietary isoflavones are believed to have health benefits. For example, they are believed to be responsible for the cholesterol-lowering effect of soy products, and may help prevent breast cancer. Moreover, isoflavones are believed to ameliorate menopausal symptoms. U.S. Pat. No. 5,972,995 teaches the treatment of cystic fibrosis patients by administering isoflavones capable to stimulate chloride transport.
Soy protein isolates are typically prepared from defatted soy meal. Proteins and soluble carbohydrates are extracted into aqueous solution (pH 7-10). The insoluble residue is mostly fiber and is removed by centrifugation. The protein is precipitated from solution as curd at its isoelectric point (about pH 4.5), further purified, neutralized, and dried. The liquid remaining after the protein has been isolated is referred to as whey and contains mainly soluble carbohydrates. Most of the isoflavones are retrieved with the protein curd.
Isoflavones also exist at the parts per million (ppm) level in the whey. Soy whey also contains carbohydrates, primarily sugars such as the monosaccharides glucose and fructose and the oligosaccharides sucrose (disaccharide), raffinose (trisaccharide) and stachyose (tetrasaccharide), in addition to proteins, salts and other bioactives. The oligosaccharides raffinose and stachyose require the enzyme α-galactosidase, which is not present in the human gastrointestinal tract, to be completely hydrolyzed into monosaccharides that can be absorbed into the blood stream. The unhydrolyzed oligosaccharides pass into the large intestine where they are fermented by anaerobic microorganisms producing gases such as CO2, H2, and CH4 that lead to flatulence. Currently, the soy whey is treated as waste with significant disposal costs. The selective removal of these undesirable oligosaccharides from soy whey would yield new, nutritious food products.
Methods for the removal of oligosaccharides from soybean wastes are known in the art. For example, Matsubara et al [Biosci. Biotech. Biochem. 60:421 (1996)] describe a method for recovering soybean oligosaccharides from steamed soybean wastewater using reverse osmosis and nanofiltration membranes.
JP 07-082,287 teaches the recovery of oligosaccharides from soybean oligosaccharide syrup using solvent extraction. The method comprises adding an organic solvent to the aqueous solution containing the oligosaccharides, heating the mixture to give a homogeneous solution, cooling the solution to form two liquid layers, and separating and recovering the bottom layer. KR 2000/055133 describe a method for separating oligosaccharides from bean curd waste solution. In that method, the waste solution is passed through a polymeric resin column to remove saponin and isoflavone. Then, the waste solution that passed through the column is filtered and concentrated to recover the oligosaccharides.
In all these disclosures, the undesirable oligosaccharides raffinose and stachyose are recovered along with the desirable sugars, sucrose, glucose, and fructose. There have been no reports of a method for selectively removing the undesired oligosaccharides raffinose and stachyose from plant processing waste products such as soy whey to obtain a nutritious product containing isoflavones, and digestible sugars, such as glucose, fructose, and sucrose.
Zeolites are high capacity, selective adsorbents that have been widely used for the separation of a variety of chemical compounds. Zeolites can be generically described as complex aluminosilicates characterized by three-dimensional framework structures enclosing cavities occupied by ions and water molecules, all of which can move with significant freedom within the zeolite matrix (Meier et al, Atlas of Zeolite Structure Types, Elsevier, 2001). In commercially useful zeolites, the water molecules can be removed from or replaced within the framework structures without destroying the zeolite's geometry.
Zeolites have been widely used as bulk adsorbents and as chromatography supports for separating a variety of substances including gases, hydrocarbons, and alcohols. The use of zeolites as selective adsorbents for carbohydrates is particularly well known in the art. For example, the use of zeolites for the separation of monosaccharides is described by Ho et al [Ind. Eng. Chem. Res. 26:1407 (1987)], and Sherman et al [Stud. Surf. Sci. Catal. 28:1025 (1980)]. Additionally, a process for separating monosaccharides using zeolite adsorbents is described in U.S. Pat. Nos. 4,405,377 and 4,483,980. The adsorption selectivity of the zeolites to monosaccharides is determined by the extent of interaction with the cations present in the zeolite, as well as the geometric constraints imposed by the zeolite pore geometries and cation positions, as discussed by Sherman et al [Stud. Surf. Sci. Catal. 28:1025 (1980)].
Buttersack et al [J. Phys. Chem. 97:11861 (1993)] report that the dealumination of Y-zeolites enhance their affinity to mono-, di-, and trisaccharides by hydrophobic interactions. The adsorption of oligosaccharides such as raffinose and stachyose by a hydrophobic zeolite, specifically, dealuminated FAU type zeolite (Si/Al=110), is described by Buttersack et al [Langmuir 12:3101 (1996)]. The FAU type zeolite used in that investigation was sold by Degussa Company, South Plainfield, N.J., under the product name Wessalith® DAY-55. That disclosure reports that DAY-55 has a very strong affinity for stachyose, a strong affinity for raffinose and sucrose, and very little affinity for glucose when tested in a single component system. The adsorption characteristics of this zeolite were not tested in a multi-component system consisting of a mixture of sugars.
There remains a need for a simple, economical process to remove undesirable oligosaccharides such as raffinose and stachyose from biological or plant processing waste products such as soy whey and other vegetable wheys to give a nutritious product containing isoflavones and digestible sugars, such as glucose, fructose, and sucrose. The adsorbent for this purpose must have a high selectivity for raffinose and stachyose in the presence of the digestible sugars and other components of the whey.